Radiofrequency activated inkjet inks and apparatus for inkjet printing

ABSTRACT

This invention relates to fast-drying RF inkjet composition and apparatus for inkjet printing RF inkjet compositions. The compositions and apparatus are useful for inkjet printing onto a variety of media including both porous and non-porous substrates. The RF inkjet composition desirably includes RF susceptors combined with polar carriers which may be activated by RF energy to generate heat within the RF inkjet composition, resulting in enhanced evaporative drying.

FIELD OF THE INVENTION

The present invention provides radiofrequency (RF) activatable inkjet inks, methods for printing the inks and apparatus for inkjet printing using RF inkjet compositions.

BACKGROUND OF THE INVENTION

Most inkjet inks currently available are low viscosity liquids which contain large amounts of water, often 90 weight percent or greater. The use of water as a solvent in these inks is advantageous because it is inexpensive, environmentally friendly and non-toxic. Unfortunately, water also has a relatively high boiling point and a high latent heat of evaporation. For this reason, inkjet inks which contain large amounts of water tend to be slow-drying. Slow-drying inks are disfavored for many printing applications because they lead to slow printing rates.

To speed up the printing rate of water-based inkjet inks, printing equipment may employ external heating devices to speed up the evaporation of water from the inks. Inkjet drying techniques include passing media with wet ink images against or near heated rollers or platens. Another approach to increasing the drying rate of water-based inkjet inks is to generate heat internally within the inks by coupling energy into the water or incorporating radiation susceptors into the ink formulations. The heating of water or susceptors is achieved by exposing the ink to radiation of a suitable frequency, causing heat to be generated within the ink and speeding up the evaporation of water and other volatile solvents. Most of these susceptors are microwave activated inorganic salts and the use of microwaves to heat and dry the water is slow and relatively inefficient. The use of external heating devices entails several complications. For example, contact of heating devices with wet ink image can produce smudging or smearing. Also, application of heat and subsequent drying of media while in contact with rollers can produce undesirable buckling or curvature of the output. This may require additional steps to flatten the media after printing and drying.

Therefore, a need exists for an improved, fast drying inkjet compositions and apparatus for inkjet printing using these compositions.

SUMMARY OF THE INVENTION

One aspect of the present invention provides fast-drying RF inkjet compositions. The RF inkjet compositions include at least one RF susceptor, a colorant and at least one polar carrier. It is believed that the RF inkjet composition generates heat when exposed to radiofrequency energies from ionic conduction caused by the movement of dissociated ions. This internally-generated heat enhances evaporation of volatile liquids from the ink compositions and results in shorter drying times. In some instances, the inkjet compositions provided herein, may be dried at a rate fast enough to allow for a printing speed of one page per second, or even faster, without any smearing of the inks.

In one embodiment of the RF inkjet composition, RF energy is used to cause a chemical reaction in the ink during drying. This produces images with improved resistance properties due to crosslinking. However the wet inks retain resolubility during printing as drying and crosslinking occur upon RF activation. Accordingly these inks cause reduced clogging of the inkjet heads.

Another embodiment of the inkjet inks provides RF inkjet compositions wherein chemical reactions are used for binding of inks to media. In one aspect of this invention, media containing reactive groups for reaction with the RF activated inkjet inks are used. Reaction between inks and media occur upon exposure to RF energy. This allows printing on non-porous media such as plastic films or modified paper substrates with improved resistance properties of images produced.

In still another embodiment of the inkjet inks, exposure of the inks to RF energy causes reaction of a polar carrier in the ink with ionomers or other components present in the inks. This produces images with improved resistance properties due to consumption or reduction in the amount of polar carrier. Another advantage of these inks is their reduced sensitivity to RF energy as the heating progresses so that these inks are self limiting and show controlled increase in temperature.

In some embodiments, the RF inkjet compositions are sublimation inks. These inks include RF susceptors and sublimation colorants in inkjet ink. In a preferred variation of this embodiment, the RF susceptor provides a polymeric binder for the sublimation colorants. The binders are used to support the colorants prior to sublimation in a transfer sublimation printing process. In one exemplary embodiment, the RF-activatable binder is an RF-activatable ionomer and the sublimation colorant is a sublimation dye. The sublimation inks may initially be provided in the form of a dispersion that is applied to a transfer substrate. Application of RF energy to the transfer substrate results in sublimation of the sublimation colorants and subsequent deposition of the colorants onto a suitable media to produce an inkjet image. The use of RF-activatable binders provides improved rate of ink transfer to the media.

In another embodiment of the invention, RF-activitable sublimation inks including RF susceptors and sublimation colorants are used for direct printing to a media such as textile or paper. RF activation is used for sublimation of dyes onto the media. This RF activation also results in heating of the media, making the media receptive to dye. The use of RF inkjet compositions results in printing with little or no distortion of the media in comparison with a thermal process that does not use RF activatable inks.

The present invention also provides RF-activatable solid hot melt inkjet inks for printing. These inks allow rapid heating of the hot melt and ejection on to the media used for printing. Examples of RF-activatable hot melts include those based on ionomers such as those from acrylic copolymers with polar carriers such as glycerol. These hot melt inks contain no or reduced amount of solvents.

In addition to inkjet inks, the present invention provides compositions for RF-activatable protective water-borne coatings, such as an overprint varnishes for application over previously applied inkjet images. The protective water-borne coatings contain ionomers and a polar carrier so that the composition can be activated by RF energy. The heating and drying of the protective coating provides a high gloss, protective barrier for the inkjet images. Similarly, the RF-activatable coatings may be applied to media prior to inkjet printing. The media so coated may then be heated using RF energy, such that the rate of drying for an ink subsequently printed onto the media is increased. This aspect allows high gloss printing applications which require coatings of higher thickness. RF-activitable inks and overprint varnishes can dry faster than those by IR or forced drying means.

A second aspect of the invention provides an inkjet printer and a method for impulse or drop on demand inkjet printing using RF energy to superheat a droplet of ink for deposition on a media. Although the RF inkjet compositions described herein are well-suited for use with the printers, other RF inkjet compositions may also be employed. The printers use an RF emitter coupled to a printing nozzle to rapidly heat an RF inkjet composition in the nozzle which generates a pressure pulse. The pressure pulse results in the ejection of an ink drop from the nozzle. The use of RF heating to create a pressure pulse has advantages over piezoelectric and thermal resistive heating-based impulse printing methods because RF heating does not require contact with the ink droplet. Therefore the RF emitter does not require changing upon changing the ink such as an ink of different color. Moreover RF heating provides a more uniform heating of the ink droplets in comparison with thermal resistive heating, resulting in less clogging of the inkjet heads. Examples of RF inkjet composition useful for this purpose include inks containing at least one RF susceptor, a colorant and a polar carrier described above.

A third aspect of the invention provides apparatus for the delivery of RF energy in inkjet printers for the purposes of drying and/or crosslinking RF inkjet inks and coatings. Although the present RF inkjet compositions and coatings are well-suited for use with these apparatus, other RF inkjet compositions and coatings may also be employed. One apparatus in accordance with the present invention provides an inkjet printer for drop on demand and continuous inkjet systems wherein one or more transistor RF modules are used to drive a low-power applicator system for inks. In some embodiments, the applicator system couples RF energy into the ink stream before its contact with the media onto which it is ultimately printed. A part of the volatile components such as water and/or other solvents are removed as a result of RF activation. The resulting inks therefore dry faster, produce less ink bleeding and higher gloss. In one embodiment of this invention the inks dry in less than one second after application on the media. Another aspect of this invention provides an inkjet printer for color printing wherein each colored cartridge is provided with separate RF modules to drive the applicator system for inks. In one embodiment of this invention, the RF module is incorporated into the ink cartridge. In another embodiment of this invention, the RF module is mounted separately on a carriage for the ink cartridge.

Another exemplary apparatus provides an RF module to couple RF energy into an image or a portion of the image immediately after printing on media. This results in volatilization of water and/or other solvents and faster drying of the RF inkjet compositions. In one embodiment of this invention the inks dry in less than one second after application on the media. In one embodiment of this invention, one or more transistor RF modules drive a low-power applicator system to couple the RF energy into the image or a portion of the image immediately after printing. In one aspect of this invention, the applicator is incorporated into the ink cartridge by utilizing the traces on the flexible printed circuit of the cartridge. Another embodiment of this invention mounts the RF applicator on the print carrier separately from the ink cartridge. Two RF applicators are desirable to provide heating of the RF activitable ink when used with multidirectional printing.

Another apparatus provides a RF power source to drive an applicator system incorporated into the roller drive used to move media through a printing zone in an inkjet printer. This apparatus couples RF energy generated in the roller of the roller drive into the image printed on the media. This produces faster drying inks due to application of RF energy shortly after printing of the media and volatilization of water and/or other solvents. In one embodiment of this invention the inks dry in less than one second after application on the media. One example of this invention includes a RF power source mounted inside the roller used for moving media. In another embodiment, the RF power source is mounted separately on the printer chassis. An interdigitated probe (IDP) system utilizing a series of alternating electrodes may be used as the RF emitter to achieve uniform drying of inks using stray field energy. In another embodiment the media passes between two electrodes after printing and drying is achieved in the through field.

Yet another apparatus provided by this invention includes a RF power source which drives an interdigitated radiofrequency applicator system. After printing with the RF inkjet composition, the media is passed over the applicator so as to couple the RF energy with the image present on the media. This causes faster drying of the image.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a suitable heating system that is capable of generating an electromagnetic field for activating the inks of the present invention.

FIG. 2 shows an interdigitated probe system for generating RF energy.

FIG. 3 shows an RF applicator that includes interdigitated probes embedded into the substrate roller of an inkjet printer.

FIG. 4 shows a through field RF applicator applied to a roller for activating RF ink on the printed medium.

FIG. 5 shows the printed medium passing over an RF applicator having a page wide interdigitated probe assembly mounted after the drive roller.

FIG. 5 a shows a block diagram of the RF field generating system that can be assembled to demonstrate and test the invention.

FIG. 5 b shows an example of the ink application equipment and the interdigitated probe assembly mounted in a paper transport mechanism.

FIG. 6 shows an inkjet cartridge that includes an interdigitated RF applicator integrated into the flex circuit of an inkjet cartridge.

FIG. 7 shows an inkjet cartridge with two RF applicators, mounted on the print head carrier on each side of the cartridge.

DETAILED DESCRIPTION

Definition of Terms

“RF Susceptors” mean either ionic or polar compounds introduced as a component into a composition such that RF heating of the resulting RF inkjet composition occurs.

“Polar Carrier” provides a mobile medium in which RF susceptors are dissolved, distributed, or dispersed. Polar carriers can be liquids such as solvents, plasticizers, and humectants. These can be organic or aqueous type

“RF Inkjet Composition” comprises at least one RF susceptor and at least one polar carrier interfaced with one and other and/or mixed or blended together. The RF Inkjet Composition will also include colorants, binders, surfactants, wetting agents, de-foamers, humectants, buffers, chelating agents, solubilizers and biocides sufficient for the performance of the inkjet ink.

“RF-activatable coating” comprises at least one RF susceptor and at least one polar carrier interfaced with one and other and/or mixed or blended together. The RF-activatable coating will also include colorants, binders, surfactants, wetting agents, de-foamers, humectants, buffers, chelating agents, solubilizers and biocides sufficient for the performance of the coating

This invention relates to fast-drying RF inkjet compositions and apparatus for inkjet printing RF inkjet compositions. The compositions and apparatus are useful for inkjet printing onto a variety of media including both porous and non-porous substrates. Specific examples of suitable media include, but are not limited to, papers, including paperboard, plastic and rubber substrates, textiles, and woven and non-woven materials. The RF inkjet composition may be activated by RF energy to generate heat within the RF inkjet composition, resulting in enhanced evaporative drying. The RF inkjet compositions include at least three components: (1) a RF susceptor; (2) a polar carrier; and (3) a colorant. The RF susceptor is an ionic or polar compound and acts as either a charge-carrying or an oscillating/vibrating component of the RF inkjet composition. The RF inkjet composition generates thermal energy in the presence of an RF field. According to the present invention, the RF susceptor can be an inorganic salt (or its respective hydrate), such as stannous chloride, zinc chloride or other zinc salt, or lithium perchlorate, or an organic salt, such as lithium acetate. The RF susceptor can be a non-ferromagnetic ionic salt. In some embodiments, the RF susceptor is an ionomer. As used herein, an ionomer is a macromolecule in which a small but significant proportion of the constitutional units have ionizable or ionic groups, or both. The ionomers in the RF inkjet compositions generate heat when exposed to radiofrequency energies from ionic conduction caused by the movement of dissociated ions in the polar liquid media. The polar media in inkjet inks comprises the water and or other additives such as solvents and humectants that have high dielectric constants suitable for dissociation of the ions.

In certain embodiments, RF susceptor can be carbon black or metallic particles. RF activation of these susceptors is via conductive heating.

The RF susceptors are present in the RF inkjet composition in amounts sufficient to provide a desired printing speed. The maximum printing speed achievable by a given inkjet ink composition will depend on the amount of RF energy converted to heat. This is a function of both the radiofrequency susceptibility of the inkjet ink as well as how balanced the RF field generating circuitry is. However too much heating may lead to arcing. The amount of RF susceptor in the RF inkjet composition may also affect viscosity of the compositions. Balancing these considerations of maximizing printing speed, avoiding arcing and optimizing viscosity, the inventors have discovered certain formulations of fast-drying, RF inkjet compositions that are well-suited for inkjet printing applications.

In some embodiments, the RF inkjet compositions contain substantial amounts of RF susceptors. For example, the RF inkjet composition may include about 0.1 to 35 wt. % of a susceptor such as an ionomer, based on the total weight of the inkjet ink composition. This includes embodiments where the inks contain about 0.5 to 10 wt. % ionomer and further includes embodiments where the inks contain about 1 to 3 wt. % ionomer, based on the total weight of the inkjet ink composition. A variety of RF-activatable ionomers may be used in the RF inkjet composition. Examples of such ionomers are described in detail in U.S. Pat. No. 6,348,679. One specific class of ionomers that may be used in the RF inkjet composition is based on acrylic acid polymers and copolymers. The copolymers are polymerized from at least one of an acrylic acid or methacrylic acid monomer and at least one additional monomer such as a vinyl aromatic monomer (e.g., styrene) or ethylene. Specific examples of such copolymers include, but are not limited to, styrene-acrylic copolymers and salts thereof, ethylene-acrylic copolymers and salts thereof and vinyl acetate-acrylic copolymers and salts thereof. The acrylic acid copolymers may be made using well-known polymerization techniques, including batch, continuous and semi-continuous polymerizations. Suitable polymers may also be made by living/controlled polymerization methods that produce narrow molecular weight distributions or alternative structures such as block copolymers. Exemplary methods include but are not limited to anionic polymerization, reversible addition fragmentation transfer or atom transfer methods. In addition, commercially available suitable acrylic polymers may be used. One example of a commercially available styrene-acrylic copolymer is JONCRYL™ 682, available from Johnson Polymer LLC, Sturtevant, Wis. From these polymers and copolymers, ionomers may be obtained by neutralization with a suitable base. In some embodiments acrylic acid polymers or copolymers have acid numbers in the range 35 to 350 mg of KOH/g and weight-average molecular weight in the range 1500 to 50,000. Suitable bases include, but are not limited to, KOH, LiOH, NaOH, Mg(OH)₂, Ca(OH)₂ and amines, including ammonia.

Other suitable acrylic acid polymers and copolymers and salts thereof are described in U.S. Pat. Nos. 5,821,294; 5,717,015; 5,719,244; 5,670,566; 5,618,876; 5,532,300; 5,530,056; 5,519,072; 5,371,133; 5,319,020; 5,037,700; 4,713,263; 4,696,951; 4,692,366; 4,617,343; 4,948,822; and 4,278,578; the entire disclosures of which are incorporated herein by reference. Examples of commercially available acrylic acid copolymers include ethylene acrylic acid copolymer and the ammonium (MICHEM™ 4983P) and sodium (MICHEM™ 48525R) salts thereof available from Michelman Incorporated, Cincinnati, Ohio. Additional examples are vinyl acetate-acrylic copolymers (e.g. ROVACE™ HP3442) available from Rohm and Hass, Philadelphia, Pa.

Maleic anhydride polymers, copolymers and salts thereof are another class of ionomer that may be used in the RF inkjet composition. Specific examples of suitable maleic anhydride-based copolymers include, but are not limited to, styrene-maleic anhydride, ethylene-maleic anhydride and propylene-maleic anhydride copolymers. Examples of this type of polymer include copolymers of styrene and maleic anhydride available under the trade name SMA™ resins from Sartomer Company, Exton, Pa.

Sulfonated polymers are another class of ionomers that may be used in the RF inkjet composition. This class includes sulfonated polyesters, copolymers and salts thereof. Also included in this group are sulfonated polystyrenes, acrylamidopropane sulfonate based polymers and urethane ionomers polymerized from a diisocyanate diol with a sulfonate functionality. More information, including specific examples of each of the above-referenced types of ionomers, may be found in U.S. Pat. No. 6,348,679, the entire disclosure of which is incorporated herein by reference. Suitable sulfonated polyesters and copolymers thereof are also described in U.S. Pat. Nos. 5,750,605; 5,552,495; 5,543,488; 5,527,655; 5,523,344; 5,281,630; 4,598,142; 4,037,777; 3,033,827; 3,033,826; 3,033,822; 3,075,952; 2,901,466; 2,465,319; 5,098,962; 4,990,593; 4,973,656; 4,910,292; 4,525,524; 4,408,532; 4,304,901; 4,257,928; 4,233,196; 4,110,284; 4,052,368; 3,879,450; and 3,018,272; the entire disclosures of which are incorporated herein by reference. Some sulfonated polyesters may be purchased commercially. Commercially available sulfonated polyesters are sold by Eastman Chemical Company, Kingsport, Tenn., under nos. AQ1045, AQ1350, AQ1950, AQ14000, AQ35S, AQ38S, AQ55S and EASTEK™ 1300.

Cationic polymers, such as those made from monomers comprising N,N-dimethylaminoethyl (meth)acrylate and the hydrogen chloride and methyl chloride salts thereof may also be used as ionomers in the present RF inkjet composition for some inks with cationic compatible formulations.

In addition to, or instead of, the ionomers, other RF-activatable compounds, such as inorganic salts, may be included in the compositions. Examples of inorganic salts useful in this invention include salts of multivalent metals such as Ca⁺², Cu⁺², Co⁺², Ni⁺², Fe⁺², La⁺³, Nd⁺³, y⁺³, or Al⁺³. Salts of these metals with anions such as nitrate, halide, acetate or sulfate can be used. However, for some applications it may be desirable to provide RF inkjet composition that are free of or substantially free of (e.g., contain no more than about 0.05, or even no more than about 0.01 wt. %) inorganic salts. The absence of inorganic salts is advantageous because salts may negatively impact the latency of the inks. Suitable inorganic salt RF susceptors and inks made from such susceptors are described in U.S. Pat. No. 5,220,346, the entire disclosure of which is incorporated herein by reference.

RF susceptors in the ink may be surfactants bound to or polymerized into polymeric binders, dispersants or ionomers present in the ink. In some cases, the RF susceptors may be the pigments or dyes used as colorants in the ink. In other embodiments, the RF susceptors are present in the media and become RF susceptible when ink contacts the substrate. One example of this is paper that contains ion containing materials. By itself, paper is only slightly RF active, however, when ink contacts the paper, the ions from the paper additives dissociate in the ink water so as to make the ink more susceptible to RF energy.

In addition to increasing the rate of evaporative drying, the RF heating of the inks may be used to initiate or increase the rate of crosslinking reaction in the inks. For example, many inks include crosslinkable binders which undergo more rapid crosslinking at elevated temperatures. By using RF susceptors in such inks, the rate of crosslinking may be increased by RF heating of the binders. In some instances, the binder itself may be an RF susceptor (e.g., an RF-activatable acrylic acid polymer or copolymer), while in other instances a polymeric binder is present in addition to an RF susceptor. A variety of crosslinkable binders are useful for RF inkjet inks wherein chemical reaction or crosslinking occurs after exposure of the ink to RF energy. Examples of suitable crosslinkable binders include those based on crosslinking reactions of epoxy groups with phenolic, hydroxyl, amine, carboxylic acid, acid anhydrides, etc. Other types of crosslinkable binders are those based on crosslinking reactions of isocyanate groups, including those of blocked isocyanate groups or those encapsulated or protected to reduce or eliminate the reaction with water. Still other types of crosslinkable binders are those based on thermosetting acrylics and appropriate crosslinking agents. Examples include acrylic copolymers containing hydroxyl groups with amino resins as crosslinking agents. Other suitable crosslinkable binders include those based on reactions of carbon-carbon double bonds. Still other crosslinkable binders also include those based on crosslinking reactions of hydroxyl group containing polymers such as polyester polyols with blocked isocyanates, amino resins, and the like.

In some instances, the crosslinkable binders are self-crosslinkable polymers, where a self-crosslinkable polymer is a polymer having a reactive functionality that is able to provide crosslinks between polymer chains via a suitable crosslinking agent that reacts with the reactive functionalities. For example, self-crosslinking binders may be polymerized from keto- or aldehyde-containing amide-functional monomers such as diacetone acrylamide (DAAM) that react with di- or polyamine or dihydrazide crosslinking agents. Other examples of self-crosslinkable monomers from which binders may be polymerized include monomers having acetoacetoxy functional groups (e.g., acetoacetoxymethacrylate (AAEM) or acetoacetoxy ethyl acrylate (AAEA) which crosslink with di- or poly-amine crosslinking agents.

In some instances the crosslinkable binders may be polymerized from carboxylic acid functional monomers, such as acrylic or methacrylic acid monomers, that crosslink with multivalent metal crosslinking agents, such as zinc or zirconium ammonium carbonate.

The polar carrier present in the RF inkjet composition may be used to dissolve or disperse binders, such as RF susceptors, and colorants and desirably also serves to enhance the RF activation of the composition. Thus, preferred embodiments of the RF inkjet composition will include a polar carrier composed of water and optionally, at least one water-miscible organic polar carrier capable of reducing the RF drying time of the compositions. The amount of polar organic carrier and water present in the compositions will depend, at least in part, on the desired RF susceptibility and viscosity of the ink formulations. As discussed above, the RF susceptibility of the formulation should be sufficient to allow for fast RF activation without arcing, while the viscosity of the formulation should be low enough to provide an ink formulation that is compatible with inkjet printing applications. In some embodiments, the RF inkjet composition will contain about 0.1 to 40 wt. % organic polar carrier. This includes embodiments where the RF inkjet composition include about 1 to 30 wt. % organic polar carrier and further includes embodiments where the RF inkjet composition include about 5 to 20 wt. % organic polar carrier. In some embodiments, the RF inkjet composition may contain about 40 to 95 wt. % water. This includes embodiments where the RF inkjet composition include about 60 to 90 wt. % water and further includes embodiments where the RF inkjet composition include about 65 to 80 wt. % water.

Solvents (including some which are polar organic carriers) that may be present in the polar carrier include, but are not limited to, ethylene glycol, propylene glycol, butylene glycol, diethylene glycol, polyalkylene glycols, glycerol, polyvinyl alcohol, amides, ethers, carboxylic acids, esters, alcohols, organosulfides, organosulfoxides, sulfones, alcohol derivatives, ether derivatives, amino alcohols, ketones, water soluble acrylic copolymers containing hydroxyl groups and mixtures thereof. In some cases, the solvents act as humectants for the ink.

The organic polar carriers used in the RF inkjet composition are desirably high boiling point solvents with high dielectric constants. For example, the organic polar carriers may have a dielectric constant of at least about 10 or even at least about 20 at 20° C. Because they have high boiling points, the organic polar carriers do not evaporate to a significant extent during ink jetting or drying. This allows for continuous RF susceptibility as the water in the ink is heated and evaporated. Specific examples of suitable polar organic carriers are listed in U.S. Pat. No. 6,348,679, the entire disclosure of which is incorporated herein by reference. Glycerol and polyethylene glycol are two examples of preferred organic polar carrier. When glycerol is present as a polar organic carrier, it may be present in an amount of about 1 to 20 wt. %, based on the total weight of the inkjet ink composition. This includes embodiments where glycerol is present in an amount of about 1 to 10 wt. % and further includes embodiments where the glycerol is present in an amount of about 1 to 5 wt. %, based on the total weight of the inkjet ink composition. In some embodiments organic polar carrier comprises formamide or N,N-dimethylformamide.

The colorants used in the RF inkjet composition may be pigments, dyes or mixtures thereof. Suitable colorants include, but are not limited to, cyan, yellow, magenta and black colorants. Examples of dyes that may be used in the RF inkjet composition include, but are not limited to, acid dyes, basic dyes, direct dyes, reactive dyes and anionic and cationic dyes. Examples of pigments that may be used in the RF inkjet composition include, but are not limited to, titanium dioxide pigments, iron oxide pigments, carbon black and organic pigments. The pigments desirably have particle sizes that are sufficiently small to avoid clogging of inkjet printer nozzles. Clogging of the nozzles may generally be avoided by employing pigments having an average particle diameter of no more than about 5 microns, and, preferably, no more than about 1 micron. The amount of colorant present in the RF inkjet composition will depend on the desired color and intensity of the final ink product. However, in some illustrative embodiments, colorants may be present in an amount of about 1 to 10 wt. % based on the total weight of the inkjet ink composition. This includes embodiments where the inkjet inks include about 1 to 8 wt. % and further include embodiments where the inkjet inks include about 2 to 6 wt. % colorant based on the total weight of the inkjet ink composition. Examples of suitable pigments and dyes for use in the present ink formulations may be found in U.S. Patent Application Publication No. US 2004/0080593 and in U.S. Pat. No. 5,814,138, the entire disclosures of which are incorporated herein by reference. Some non-limiting examples of dyes that may be used in the present RF inkjet composition include anthraquinones, monoazo dyes, disazo dyes, phthalocyanines, aza[18]annulenes, formazan copper complexes, triphenodioxazines, Bemacid Red 2BMN; Pontamine Brilliant Bond Blue A; Pontamine; Food Black 2; Direct Blue 199; Direct Blue 86; Reactive Red 4; Acid Red 92; Cartasol Yellow GTF Presscake, available from Sandoz, Inc.; Acid Yellow 23; Basacid Black X34, available from BASF, Carta Black 2GT, available from Sandoz, Inc.; Direct Brilliant Pink B (Crompton-Knolls); Levaderm Lemon Yellow (Mobay Chemical Company); Spirit Fast Yellow 3G; -Sirius Supra Yellow GD 167; Pyrazol Black BG (ICI); Morfast Black Conc A (Morton-Thiokol); Diazol Black RN Quad (ICI); Direct Yellow 86; Acid Red 249); Direct Black 168; Direct Yellow 132; Aminyl Brilliant Red F-B, available from Sumitomo Chemical Co. (Japan) and mixtures thereof. Some non-limiting examples of pigments that may be used in the compositions include titanium dioxide, iron oxide, carbon black, copper tetra-4-(octadecyl sulfonamido) phthalocyanine, X-copper phthalocyanine pigment, CI Pigment Blue, Anthradanthrene Blue, Special Blue X-2137, diarylide yellow 3,3-dichlorobenzidene acetoacetanilides, CI Solvent Yellow 16, CI Dispersed Yellow 33, 2,5-dimethoxy-4-sulfonanilide phenylazo-4′-chloro-2,5-dimethoxy aceto-acetanilide, and Permanent Yellow FGL. In some cases the colorants themselves may provide sufficient ions to promote adequate ionic conduction in the presence of the RF energy.

Optionally, a polymeric binder may be added to the RF inkjet composition. These polymeric binders act as viscosity control agents and help to affix colorant to a substrate after the ink has been printed and dried. However, it should be recognized that when RF-activatable ionomers are present, the RF-activatable ionomers themselves may provide an adequate binder, eliminating the need for additional polymeric binder. Where a polymeric binder is present, the amount of polymeric binder added should be sufficient to provide an inkjet ink composition having a viscosity suitable for inkjet printing applications. For example, some RF inkjet composition in accordance with the present invention will contain about 0.1 to 20 wt. % polymeric binder. This includes embodiments where the RF inkjet compositions contain about 1 to 10 wt. % polymeric binder. Suitable polymeric binders are listed in U.S. Patent application publication No. 2004/0080593. These polymeric binders include water soluble polymers such as gum Arabic, polyacrylate salts, polymethacrylate salts, polyvinyl alcohols, hydroxypropylcellulose, hydroxyethylcellulose, polyvinylpyrrolidinone, polyvinylether, starch, polysaccharides and polyethyleneimines derivatized with polyethylene oxide and polypropylene oxide, water-soluble acrylic resins and emulsion polymers.

Other additives that may be found in RF inkjet composition include, but are not limited to, surfactants, wetting agents, defoamers, humectants, buffers, chelating agents, solubilizers and biocides. These additives may be present in amounts of about 0.1 to 15 wt. % (e.g., about 0.5 to 10 wt. %), based on the total weight of the ink composition. More specifically, a typical ink composition might contain about 0.1 to 10 wt. % surfactant, about 0.05 to 1 wt. % biocide, about 0.1 to 0.5 wt. % buffer and/or about 0.01 to 1 wt. % other additives, such as chelating agents, defoamers and solubilizers.

The inkjet inks are characterized by high ionic conductivities and RF energy susceptibilities. Ionic conductivity is not the same as electric conductivity as highly ionic conductive materials may be substantially electrical insulators. One measure of radiofrequency susceptibility is the dielectric loss factor otherwise known in the art as the imaginary portion of the relative permittivity. In contrast, the real portion of the relative permittivity of a material is known as the dielectric constant. For some of the inkjet inks provided herein the dielectric loss factor is at least about 15. This includes embodiments where the dielectric loss factor at a frequency of 30 MHz is at least about 15, further includes embodiments where the dielectric loss factor at a frequency of 30 MHz is at least about 100, still further includes embodiments where the dielectric loss factor at a frequency of 30 MHz is at least about 1000, even further includes embodiments where the dielectric loss factor at a frequency of 30 MHz is at least about 10,000.

In other embodiments, ions required for promoting ionic conduction in the radio frequency field are additionally provided by the substrate itself. The act of printing the ink onto the substrate causes these ions to dissociate in the water and polar solvent, thus providing sufficient ionic character for ionic conduction. In some embodiments, the colorant itself may be active in the RF field. For example, some carbon black pigments are electrically active as a result of their structure. Inks containing these materials can be a problem unless the RF susceptibility is controlled so as to prevent arcing in the field.

The inkjet inks may be formulated such that they have viscosities and surface tensions that are appropriate for inkjet printing applications. Viscosities of about 1 to 25 cp at 25° C. and surface tensions of about 30 to 60 dynes/cm at 25° C. are typically suitable for inkjet printing. However, the present compositions are not limited to those having viscosities and surface tensions that fall within these ranges.

In one embodiment of the invention, the RF inkjet compositions are sublimation inks. These inks include a RF susceptor and a heat activated sublimation colorant. The RF susceptor may be a RF-activatable ionomer, as described herein. However other RF susceptors, including metal salts, may also be used. The sublimation colorant may be any dye, pigment or coloring agent that is caused to sublimate at an elevated temperature. As used herein, an elevated temperature is any temperature above the printing temperature at which the inks are printed onto a transfer substrate. For typical inkjet printers, the elevated temperatures will generally be at least about 300° C. (e.g., about 300 to 500° C.). Sublimation colorants are known and commercially available. The sublimation colorants may have a sublimation temperature or activation temperature of 120° to 300° C. Examples of such colorants are the following dyes: C.I. Disperse 3 (cyan), C.I. Disperse 14 (cyan), C.I. Disperse Yellow 54 (Yellow), C.I. Disperse Red 60 (Red), Solvent Red 155 (Diaresin Red K, red), etc. Commercial sources of such dyes include Keystone Aniline's Sublaprint® series, BASF Corporation's Bafixan® Transfer Printing dyes series, Eastman Chemical's Eastman® disperse series and Crompton & Knowles Corporation's Intratherm® disperse dyes series.

The sublimation inkjet inks may further contain a solvent, which may be water, an organic solvent or a mixture of water and at least one other organic co-solvent. Suitable solvents include the polar carriers and solvents described previously. In formulating the inkjet inks, it may be desirable to first prepare a dispersion of the sublimation colorant in the solvent, with or without the help of a dispersion aid. Sublimation colorant dispersion are commercially available in the form of ink formulations. These ink formulations may be combined with the RF susceptor and any optional binders and additives to provide an RF-activatable sublimation ink. Examples of sublimation ink formulations that may be used to make the present RF-activatable sublimation inks include those described in U.S. Pat. Nos. 5,487,614; 5,488,907; 5,601,023; 5,640,180; 5,642,141; 5,734,396; and 5,830,263; the entire disclosures of which are incorporated herein by reference.

In addition to the RF susceptors, the sublimation colorant, solvents and dispersants, the sublimation inkjet inks may include various additives commonly found in inkjet inks. These include, humectants, biocides, surfactants, complexing agents, and other similar components. The RF inkjet inks of the present invention can be provided in water soluble, water dispersible, water reducible or emulsion forms.

Methods of printing with the RF-activatable sublimation inkjet inks include the steps of applying the inks to a transfer substrate and activating the inks by exposing them to RF radiation to generate heat in the inks, causing the sublimation colorants to sublime (vaporize) and deposit onto the media to be printed where they transform back into solids. Media that may be advantageously printed using a sublimation printed process include, but are not limited to, papers, textiles and non-woven materials. The sublimation inks may be printed onto the transfer substrate by a printer, but the sublimation colorants are not sublimated during this initial printing process. However, in some embodiments the sublimation inks may undergo RF heating during the initial printing step in order to speed up the drying of the sublimation ink composition on the transfer substrate. Using RF energy to sublime the sublimation colorants provides faster heating than conventional heat presses and, therefore, greatly increases the rate of ink transfer.

In another embodiment of the invention, the RF inkjet composition react with media used for printing. In one aspect of this invention, media containing reactive groups for reaction with the RF inkjet composition are used. The reactive component in the ink may be the RF susceptor itself, or another reactive component, such as a binder, that is heated by an RF susceptor. In some embodiments, the reactions occur between binders and/or polymeric RF susceptors in the inks and the media upon exposure to RF energy. This allows printing on non-porous media, such as plastic films, with improved resistance properties of images produced. For example, plastic films that are corona treated can be used with inks based on multivalent metal crosslinkers (e.g., zinc or zirconium ammonium carbonate) and carboxylic acid functional binders. Similarly, paper containing alkyl diketene can be used for inks containing binders with cationic or amine functionalities. In other embodiments, the colorant used in the RF inkjet composition has a reactive group that reacts with a medium, such as a textile, to fix the colorant to the medium. These “reactive colorants” are well-known. In many instances, the reactivity of the colorant depends on the temperature. Therefore, the use of RF heating, as provided by the RF susceptors described herein, may be useful in promoting reactions between reactive colorants and media.

RF-activatable hot melt inks are also provided. In their basic form these inks include an RF susceptor (e.g., a metal salt) and a colorant, such as a dye or pigment. The hot melt inks are printed in a molten state at an elevated temperature and then allowed to yield a printed image. The hot melt inks may be heated to a molten state by activating the RF susceptors contained within the hot melt inks.

In other embodiments of the invention, a coating is provided on the media to be printed. The coating may be applied to the media prior to printing or it may be applied over the ink after printing, as in the case of an overprint varnish. The RF susceptor may be present in the ink, the coating or both. In some embodiments, the coating includes an RF susceptor. In these embodiments RF heating of the coating may be used to increase the evaporative drying of the coating and/or an ink printed over or under the coating. In some embodiments, the coating includes functional groups that are capable of reacting with (e.g., crosslinking with) at least one component in the inks and the reaction is initiated by, or the rate of the reaction is increased by, RF heating resulting from the activation of the RF susceptor in the ink and/or the coating. For example, the RF heating initiated or facilitated reactions may occur between reactive functionalities in the coating and reactive functionalities on polymeric binders or reactive colorants in the inks. Media coatings for inkjet printing are known and commercially available. RF susceptors, including those described herein, may be added to such coatings to provide RF-activatable media coatings. Examples of media coatings (or “fixers”) to which the RF susceptors may be added to provide RF-activatable over- and under-print varnishes are described in U.S. Pat. No. 6,443,568, the entire disclosure of which is incorporated herein by reference. By activating the RF susceptors, the coatings are heated, which may increase the rate of ink drying on the coated media and/or initiate or increase the rate of reactions between the coatings and the inks and/or cause the coating to flow and provide high gloss and optionally barrier properties. Suitable RF-activitable media coatings are described in U.S. Pat. No. 5,500,668, the entire disclosure of which is incorporated herein by reference.

The present RF inkjet inks can also be formulated with polymers made by controlled polymerization processes such as reversible addition fragmentation transfer (RAFT) polymerization. These polymers can be presence as or in addition to the components of RF inkjet inks discussed above.

In one other embodiment of the invention, a surfactant or polar carrier present in the RF activitable inkjet ink undergoes a chemical or physical change upon RF activation of the ink. This provides printed media with increased resistance properties.

RF drying of the RF inkjet compositions provided herein may be accomplished by printing the inks onto a substrate and exposing the printed inks to RF energies suitable for activating the RF-activable materials. As used herein, the term “RF” may include any frequencies that fall within the RF range of the electromagnetic spectrum. In some embodiments the RF susceptors are activatable at radiofrequencies of about 100 kHz to 5.0 GHz. In some embodiments it is advantageous to use RF susceptors that are activatable at frequencies of less than about 3 GHz or even less than about 1 GHz, that is, at frequencies lower than microwave frequencies (i.e., about 1 GHz to 300 GHz). One standard preferred frequency is 27.12 MHz which is an international industrial, scientific and medical band. Other standard frequencies exist as well and may be preferred but not required for this invention. Without wishing or intending to be bound to any particular theory of the invention, the inventors believe RF wavelengths below the microwave frequency range may be superior because at these frequencies, the heat generated due to ionic conductivity is much more significant than at the higher microwave frequencies where dielectric type dipole rotation dominates as a source of heat. Otherwise stated, at radiofrequencies below microwave frequencies ionic conductivity dominate the loss mechanism whereas at microwave frequencies the relaxation process (i.e., the reorientation of permanent dipoles) is more important. Equipment that may be used to expose the printed inks to the appropriate RF energies is known and commercially available.

The power of the radio frequency field to which the inks are exposed generally ranges from about 1 to 1000 W for home and office printers. However, a higher power may be desirable for industrial printing devices. Thus, in some embodiments the ink compositions are exposed to radiofrequency fields with a power of about 50 to 5000 W. This includes embodiments where the ink compositions are exposed to radiofrequency fields with a power of about 200 to 4000 W.

Since ink tends to spread, wet and penetrate porous substrates, the print characteristics can be manipulated by adjusting the time between jetting of ink on the substrate surface and heating by radio frequency energy by locating the RF field source at a selected distance from the jetted ink to obtain desired properties. For example one can put the field closer to the ink to increase drying on the surface of the substrate or to increase time by increasing the distance to the field so as to allow further penetration and spreading of the ink into and on the substrate. Additionally, this can be accomplished by providing a plurality of RF heating elements in series/parallel combination, each of which can be independently controlled in terms of RF power and frequency. In such a case, by selectively turning off or on certain elements, the distance between the ink contact point with the substrate and the first powered RF element can be manipulated. In these cases, the time the ink has to spread or penetrate the substrate can be adjusted and the ink bound to the substrate or dried at various locations in the substrate as desired to obtain the required print quality such as color intensity and/or strikethrough.

In some cases the RF field can be used to preheat the substrate prior to contact with the ink to improve printing characteristics and optionally post heating the substrate and the ink print in a radio frequency field. In some preferred cases, the preheating of the substrate such as porous paper material pre-dries and heats the sheet so that the subsequent ink print dries faster.

Generally, the inkjet inks of the present invention may be heated (i.e., activated) by any system capable of generating an electromagnetic field of sufficient strength and frequency. Examples of suitable systems are described in U.S. Pat. No. 6,348,679, the entire disclosure of which is incorporated herein by reference. FIG. 1 illustrates a high level block diagram of a suitable heating system 100 that is capable of generating an electromagnetic field for activating the inks of the present invention. Heating system 100 includes an alternating voltage generator 102 and a probe 104, which is connected to an output terminal 101 of voltage generator 102. Voltage generator 102 alternately positively charges and negatively charges probe 104, thereby creating an electromagnetic field 106 centered at probe 104. Heating can occur when an RF inkjet composition 110 is placed in proximity to probe 104. How quickly and how much heating occurs depends on the ink itself, the strength of the electromagnetic field at the ink, and the frequency of the alternating voltage 109 produced by voltage generator 102.

Generally, probe 104 is a conductive material, such as, but not limited to, copper, brass, aluminum, or stainless steel. Generally, probe 104 can have a variety of shapes, including cylindrical, square, rectangular, triangular, etc. Generally, probe 104 can be straight or non-straight, such as curved. The preferred characteristics of probe 104 ultimately depends on the application that it is being used for.

FIG. 2 illustrates one specific embodiment of a suitable system for generating RF energy. This system is termed an “interdigitated probe system.” The interdigitated probe system 201 is advantageous because it provides an extended activation zone, as shown by the dotted rectangle 250. Interdigitated probe system 201 includes a first element 202 and a second element 204. The first element 202 includes a first conductor 210 and one or more second conductors 222 connected to the first conductor 210. Preferably, conductors 222 are coplanar and uniformly spaced apart, but this is not a requirement. Additionally, in one configuration of element 202, each conductor 222 forms a right angle with conductor 210, but this is also not a requirement. Similarly, the second element 204 includes a first conductor 212 and one or more second conductors 220 connected to the first conductor 212. Preferably, conductors 220 are coplanar and uniformly spaced apart, but this is not a requirement. Additionally, in one configuration of element 204, each conductor 220 forms a right angle with conductor 212, but this is also not a requirement. In one embodiment, first element 202 and second element 204 are orientated such that conductors 220 are coplanar with conductors 222 and each conductor 220 is adjacent to at least one conductor 222.

RF applicator systems that use RF emitters, such as the probes and interdigitated probes of the type described above, may be incorporated into inkjet printing systems in order to facilitate printing with RF inkjet compositions. Such systems have the advantage of providing high power RF drying systems with short drying cycles. FIG. 3 shows an embodiment of a RF applicator 300 that includes interdigitated probes 302 embedded into a substrate roller 304. Many inkjet printers already have roller drives that include one or more rollers used to guide a medium, such as paper, through the printing zone. In the embodiment shown in FIG. 3, the roller includes two nested cylinders. The outer cylinder 304 is a rotating drive cylinder that propels the medium 308 (here, a roll of paper) through the printing zone and the inner cylinder 306 is a stationary cylinder having a plurality of interdigitated probes running longitudinally along its outer surface. Although the length of the probes may vary, they are desirably long enough to expose the entire width of the medium to RF radiation. The RF power source for the probes could be mounted inside the inner cylinder. Alternatively, the power source could be mounted external to the roller.

In one variation of the above-described RF applicator system, both the upper and lower surfaces of the printed medium may be exposed to RF radiation simultaneously. In this embodiment one surface (e.g., the lower surface) of the medium is exposed to RF radiation from an interdigitated probe system embedded in a roller, as described above. In addition, a second RF source is located above the medium. This second RF emitter may be spaced apart from, and shaped to conform to, the curvature of, the roller. The second RF emitter and may also be an interdigitated probe system wherein the probes are aligned in a substantially parallel arrangement above the upper surface of the medium, provided that the polarity of the probes is such that the electromagnetic fields above and below the medium do not cancel each other out.

Another example of an RF applicator incorporating one of the printer rollers is shown in FIG. 4. In this design, a parallel plate electrode system is created by the conductive plates 402 and 404 disposed on opposite sides of a printed medium 406. The first conductive plate 402 is a curved plate electrode disposed over the printed medium. The second conductive plate 406 is a roller electrode under the printed medium. Capacitor plates 408 may be used to couple RF energy into the roller electrode. Plates 402 and 404 are preferably constructed of copper, but may be constructed of any suitable conductive material. The printed medium 406 may be stationary or moving when exposed to the activation region between plates 402 and 404.

Instead of being embedded in a roller, the RF emitters may be incorporated into a plate positioned past the printing zone. FIG. 5 shows one such embodiment where the RF emitter is an interdigitated probe assembly 502 built into the upper surface of a horizontal plate 504 which supports the printed medium. Although the length of the probes may vary, they are desirably long enough to expose the width of the printed medium to RF radiation. In some embodiments the horizontal plate is the printer platen. In other embodiments the horizontal plate may be purchased separately from the printer and designed to mount to the printer. In still other embodiments, the plate is a stand-alone plate. These RF applicators provide physical separation between the print zone and the RF activation zone and are easily adapted to for use with existing inkjet printers.

FIG. 5 a shows a block diagram of the RF field generating system that can be assembled to demonstrate and test the invention. The RF generator 510 is an Ameritherm 27.12 MHz variable frequency 1 KW output part number 312-0028 (Ameritherm, Rochester, N.Y.) connected to the impedance matching circuit 512 using a 50 ohm cable. The interdigitated probe assembly 514 is constructed in accordance with the principles described in U.S. Pat. No. 6,812,445, with 3 mm spacing between the electrodes. In the interdigitated probe assembly shown in cross-section in FIG. 5 a, the conductors 516 of the first element 518 are oriented parallel to the conductors 520 of the second element (not shown). The interdigitated probe assembly used in this example has an RF field activation zone that is 50 mm wide and 75 mm long, and a capacitance of 23.7 picofarad. The impedance matching circuit consists of variable parallel plate capacitors of 15.2 picofarad with a parallel resonating copper tube coil of 1.4 micro henries. All connections within the impedance matching circuit and to the interdigitated probe assembly are kept short to minimize stray capacitance and inductance. During the initial start up of the system it is necessary to match the impedance of the interdigitated probe assembly loaded with the wet ink to the 50 ohm source impedance of the RF Generator. An Agilent model E4991A impedance analyzer (Agilent Technologies, US) is temporarily connected to the 50 ohm connector on the impedance matching network 512 and the adjustable capacitors are adjusted to achieve an impedance match of 50 ohms, zero phase at 27.12 MHz.

FIG. 5 b shows an example of the ink application equipment and the interdigitated probe assembly 522 mounted in a paper transport mechanism. Other methods of applying the ink may be used including the correct placing of ink jet cartridges or ink jet heads. In this example parts available from the Lee Company (USA) were assembled to provide a rate controllable ink application method. A 2 oz polypropylene Nalgene (Nalgene, USA) screw top jar 522 is pressurized with a clean dry supply of air at a pressure between 0.5 psi and Spsi. The air may be supplied through an air inlet line 524 attached to a bulk head fitting 526 extending through a cap 528 on the jar 522. Ink 530 from the jar is supplied to a solenoid valve 532 (e.g., through soft walled tubing 533) which provides the ink to an atomizing nozzle 534. Ink from the nozzle 534 may be sprayed onto a paper substrate 538 traveling under the nozzle in a given direction of travel 540. The applied ink 542 may be activated from below by the RF field 544 of the interdigitated probe assembly 514. The pressure of the atomizing air supply 536 and the ink application air supply are regulated and adjusted to provide the desired coating weight. The solenoid valve is used to turn on and off the supply of ink to the nozzle and the valve may be controlled by a paper present sensor.

In one embodiment, a Cyan inkjet ink with an e″ (dielectric loss factor) equal to 12,000, applied to a thin substrate material at an equivalent coverage volume of 0.6 milliliters per 8.5×11 in. of substrate, will change the capacitance of the interdigitated probe assembly 514 from 23.7 picofarads to 24.4 picofarads. The impedance matching circuit 512 is adjusted to maximize the energy transfer efficiency to the ink. For example, if the desired resonant frequency is 27.12 MHz and the impedance needs to be 50 ohms at 0° phase angle, the inductor located within the impedance matching network 512 will have a value of approximately 1.3 microhenrys and the capacitors within the impedance matching network 512 will have a value of approximately 18 picofarrads. This example of optimizing the energy transfer efficiency to the ink by adjusting the RF impedance matching network 512 to the capacitance of the interdigitated probe assembly 514 has been successfully tested with applied RF power ranging from 20 W up to 400 W at a frequency of 27.12 MHz and achieving an energy transfer efficiency of 85% as measured by the −3 db Q value efficiency method disclosed in US patent application number (JD 586).

To implement this invention into an inkjet printer, it is necessary to balance the susceptablity of the RF active inks to the applicator RF circuit characteristics. The RF circuit characteristics are constrained by the physical constraints of the systems described in FIG. 3 through FIG. 7. Specifically the capacitance of the probe system is restricted by the physical constraints of the RF electrode applicator system location within the inkjet printing system. Adjusting the RF susceptibility of the RF active inks to maximize the delivery of energy from the RF field to the inks is achieved through the formulation of the RF active inks composition by adjusting the ionomer or other susceptor component levels within the formulation.

Examples of apparatus suitable for fast-drying RF inkjet compositions of this invention include an RF module to drive an applicator system that couples energy into the ink stream before contact of ink with the media. In one embodiment of this invention, the probe could be incorporated into the ink cartridge. In another embodiment of this invention, the probe could be mounted separately on a carriage for the ink cartridge.

In one exemplary embodiment of an applicator system in accordance with the present invention, an inkjet printing assembly includes an inkjet cartridge and an RF applicator for heating localized areas of media along print lines so that RF inkjet compositions, upon ejection from the inkjet cartridge and after contact with the media, are exposed to RF heating substantially immediately, increasing the rate of evaporative drying of the inks. The RF applicator may include an RF emitter incorporated into the ink cartridge itself or it may include at least one RF emitter that travels with the cartridge. In a preferred embodiment, at least one RF emitter is attached to the trailing edge of the cartridge such that it activates the RF inkjet composition substantially immediately after the ink is deposited onto the medium. In a still more preferred embodiment, the ink cartridge includes a first RF emitter mounted to the cartridges leading edge and a second RF emitter mounted to the trailing edge of the cartridge. This latter construction allows for post-application RF heating of a RF inkjet composition in a bi-directional type inkjet print head. In addition, the print substrate may itself include, or be coated with, an RF susceptor. In this latter embodiment, an RF emitter associated with the leading edge of the print cartridge may be used to preheat the substrate in order to speed up the evaporative drying of the ink that is subsequently applied thereto. In an alternative embodiment, the RF emitters may travel along with the print cartridge without being attached directly to that cartridge. For example, the RF emitters may be slidably mounted to the same guide shaft to which the cartridge is slidably mounted. If more than one cartridge is present in the print head, each cartridge may be supplied with its own RF emitter or emitters or a single RF emitter, or pair, or set of emitters may be provided for the entire print head.

FIG. 6 generally shows an inkjet cartridge that includes an RF applicator in accordance with the present invention. The cartridge 600 may be slidably mounted on a guide shaft (not shown) on which it traverses back and forth across over a medium, such as a sheet of paper. The cartridge has a bottom surface 602 that remains substantially parallel to the medium surface 604 during printing. A motor-driven device such as a band or belt is mechanically coupled to cartridge to drive it back and forth on the guide shaft. In the embodiment shown in FIG. 6, the RF applicator comprises an RF probe that is built into the flexible circuit 606 on the ink cartridge. These flexible circuits (or “flex circuits”) are standard components of most inkjet ink cartridges. In this embodiment the RF applicator comprises of an RF module, impedence matching network and an RF probe mounted on the ink jet cartridge with a source of DC power from the printer chasis.

FIG. 7 shows an alternative embodiment where the ink cartridge 700 has two RF applicators 702, 704, one attached to each side of the cartridge. As shown in the embodiment of FIG. 7 each RF applicator 702, 704 includes an RF probe 706, 708 facing the surface of print substrate 710 while being proximately spaced there from. Normally, the RF emitters are mounted on planar surfaces of the applicators and oriented substantially parallel to the surface of the medium to be printed, generally, but not necessarily, at an elevation of about 2 millimeters or less above the print lines. In this embodiment the RF applicator comprises of an RF module, impedence matching network and an RF probe with a source of DC power from the printer chassis.

The RF emitters are desirably low power emitters that are incorporated directly onto the ink cartridge by utilizing available traces on the flexible circuit of the ink cartridge, as shown in FIG. 6. In one embodiment of the invention, the traces on the flexible circuit are used to provide an interdigitated probe radiofrequency emitter, of the type described above. The circuit traces used to convey the printing signals to the ink jet head are configured on the printed circuit or flex circuit in the form of an interdigitated probe of the correct dimension to sufficiently couple energy into the RF ink on the print medium. The RF power and the print signals are multiplexed into the circuit traces allowing both the correct operation of the printing and the heating of the RF ink.

Operation of the systems of FIGS. 6 and 7 will now be generally described. The ink cartridge prints swaths of ink drops across the surface of a medium as it moves both back and forth along its guide shaft. In each swath, ink dots are printed in a print line. The RF applicator or applicators pass directly over each print line on the surface of the medium as the cartridge deposits an RF inkjet composition onto the surface of the substrate. As the applicators pass over each print line, the RF emitters activate the inks, causing internal RF heating, which hastens evaporative drying and or crosslinking reactions within the inks. In the embodiment shown in FIG. 7, the leading RF applicator may be used to activate the surface of the medium in localized areas ahead of each print line, provided the substrate incorporates or is coated with an activatable coating or varnish. The trailing RF applicator begins drying each print line substantially immediately after ink is applied. Accordingly, the systems of FIGS. 6 and 7 function to dry printed lines before ink droplets forming the lines can bleed substantially into the substrate, or merge with adjacent ink droplets, or cause cockling.

Due to the proximity of the RF emitters to the printed medium, the RF inkjet compositions may be activated after they leave the printing nozzles and prior to contact with the medium, causing some of the volatile solvents (e.g., water) in the inks to be removed prior to impact on the medium. As a result, reduced ink bleed and higher gloss can be achieved because the ink has less time to migrate into porous substrates.

The inkjet printers used to apply the RF inkjet compositions of the present invention include drop on demand or impulse printers and continuous printers. One aspect of the invention provides an impulse inkjet printer where RF heating, instead of conventional resistive heating, is used to eject the inks from the nozzles in a print head. These RF activated impulse printers are modeled after conventional impulse printers where each printing nozzle in the printer is equipped with a resistive heating element or a piezoelectric element. In conventional impulse inkjet printing, a rapid pressure impulse is created in the print nozzle by either rapid heating (in the case of a thermal impulse) or rapid deformation of the piezo element (in the case of piezo impulse). As a result, a bubble of vapor is created by the excess pressure and this bubble catapults an ink drop out of the nozzle and onto the media. In the RF activated impulse inkjet printers of the present invention, the resistive heating element on the nozzles of an inkjet printer are replaced by RF emitters. These emitters are used to cause rapid superheating of a RF inkjet composition inside the nozzle. This rapid heating creates a pressure pulse and a vapor bubble which catapults an ink drop out of the nozzle.

Suitable RF emitters that may be mounted to the inkjet nozzles of an inkjet printer include RF probes, such as those described in U.S. Pat. No. 6,348,679, the entire disclosure of which is incorporated herein by reference. For example, the RF applicator may be either a miniaturized shaped interdigitated electrode system or a miniaturized shaped parallel plate electrode system as determined by the optimum energy transfer required for the ink jet system.

The use of an RF heater, rather than resistive heating, is advantageous because RF heating is a non-contact method of heating that provides uniform heating of the ink and lower cogation of the ink heads.

The RF activitable inkjet inks and apparatus described above can be used in a wide variety of printing locations, devices and applications, including, but not limited to, homes, offices, narrow and wide format commercial printing, for printing labels, barcodes, in photo kiosks, and the like.

The following illustrative embodiments are intended to further exemplify the RF inkjet compositions. These embodiments should be not interpreted as limiting the scope of the inkjet inks disclosed herein.

EXAMPLES Example 1 Preparation of a Radiofrequency-Activatable Styrene-Acrylic Ionomer

This example describes the production of a styrene-acrylic resin ionomer for use in radiofrequency-activatable RF inkjet composition. The solution was prepared from a commercially available styrene-acrylic resin, JONCRYL™ 682. JONCRYL™ 682 is a low molecular weight styrene-acrylic resin available from Johnson Polymer, Sturtevant, Wis. JONCRYL™ 682 has an acid number of 238 mg KOH/g, glass transition temperature of 56° C. and a weight average molecular weight of approximately 1700. An amount of 25.424 parts by weight of JONCRYL™ 682™ was neutralized with 7.926 parts of a 85 wt. % active potassium hydroxide pellets and 66.649 parts water. The resulting neutralized batch was then heated to 80° C. under nitrogen and agitated for 3 hours until a clear mixture was obtained. The resulting solids of the mixture was 30% and pH of 13.3.

Example 2 Preparation of RF Inkjet Composition from a Styrene-Acrylic Ionomer

This example describes the production of radiofrequency-activatable black inks from the ionomer of Example 1. Six inkjet inks are made from a commercially available ink to which the ionomer of Example 1 is added. Three inks also include glycerol as an organic polar carrier. The ink formulations (I1-I6) for each of the six inkjet inks are provided in Table 1. The amount of each component in Table 1 is given in wt. %, based on the total weight of the ink formulation. TABLE 1 INK FORMULATIONS I1 I2 I3 I4 I5 I6 Water 50-90  43-77 38-68 50-90   43-77   38-68 Isopropanol 0-15  0-13  0-11 0-15   0-13   0-11 Butylene Glycol 0.1-15   0.75-13   0.75-12   0.1-15   0.75-13  0.75-12 Glycerol 0  0  0 1.3  3.8 6.3 Ionomer 5 15 25 3.75 11.25 18.75 Carbon Black 1-15 0.5-13  0.75-7.5  1-15 0.5-13  0.75-7.5

Example 3 Preparation of RF Inkjet Composition from a Styrene-Acrylic Ionomer

This example describes the production of radiofrequency-activatable inks from the ionomer of Example 1. Six inkjet inks are made from a commercially available ink to which the ionomer of Example 1 is added. Each of the inks include glycerol as an organic polar carrier. The ink formulations (I7-I12) for each of the six inkjet inks are provided in Table 2. The amount of each component in Table 2 is given in wt. %, based on the total weight of the ink formulation. TABLE 2 INK FORMULATIONS I7 I8 I9 I10 I11 I12 Water 70-90  60-77  53-68  70-90  60-77 53-68  Isopropanol 0-5  0-4  0-4  0-5  0-4 0-4  Ethylene Glycol 0-10 0-9  0-8  0-10 0-9 0-8  Glycerol 5-10 5-10 5-10 6.5-11.5   8-12.5 10-14  Ionomer 5 15 25 3.75 11.25 18.75 Water Soluble Dye 5-10  4-8.5  4-7.5 5-10   4-8.5  4-7.5

Example 4 Preparation of RF Inkjet Composition from a Styrene-Acrylic Ionomer

This example describes the production of crosslinking radiofrequency-activatable inks from the ionomer of Example 1. Four inkjet inks are made from a commercially available binder with which the ionomer of Example 1 crosslinks upon exposure to RF energy. Each of the inks include glycerol as an organic polar carrier. The ink formulations (1-4) for each of the six inkjet inks are provided in Table 3. The amount of each component in Table 3 is given in wt. %, based on the total weight of the ink formulation. TABLE 3 WATERBORNE CROSSLINKING INK FORMULATIONS #1 #2 #3 #4 DI Water 60-90 60-90 60-90 60-90 Isopropanol 0-5 0-5 0-5 0-5 Surfactant 0.1-10  0.1-10  0.1-10  0.1-10  Glycerol 1-5 1-5 1-5 1-5 Ionomer 5 5 5 5 JONCRYL ™ 1980¹ 0.1-5   0 0 0 JONCRYL ™ 89² 0 0.1-5   0 0 JONCRYL ™ 60³ 0 0 0 0.1-5   EPI-REZ 3515⁴ 0 0.1-5   0 0.1-5   JONCRYL ™ 540⁵ 0 0 0.1-5   0 CYMEL ™ 1172⁶ 0 0 0.1-5   0 Water Soluble Dye  5-10  5-10  5-10 0 Carbon Black 0 0 0  1-15 ¹A self-crosslinkable emulsion, Johnson Polymer LLC, Sturtevant, WI. ²A styrene acrylic emulsion polymer, Johnson Polymer LLC, Sturtevant, WI. ³Solution of a styrene acrylic resin, Johnson Polymer LLC, Sturtevant, WI. ⁴An epoxy resin, Resolution Performance. ⁵A thermosetting acrylic emulsion, Johnson Polymer LLC, Sturtevant, WI. ⁶An amino resin crosslinker, Cytec Industries, West Patterson, NJ.

Example 5 Preparation of RF-Activatable Hotmelt Ink from a Styrene-Acrylic Ionomer

This example describes the production of a radiofrequency-activatable hot melt ink. A hot melt ionomer that is RF active is made by dissolving 6.61 parts by weight of potassium hydroxide pellets (85% active) in 23.4 parts of glycerol. After dissolving under heating, 23.4 parts by weight of JONCRYL™ 690 is added and reacted over 2 hours at 130° C. to form a viscous molten material. A soluble dye is then added to mixture which is allowed to cool to form a solid hot melt that is active to RF energy. The amount of each component in Table 4 is given in wt. %, based on the total weight of the ink formulation. This ink is particularly useful for printing labels and barcodes. TABLE 4 Amount Ionomer in a polar carrier 80-99 Dye  1-20

The invention has been described with reference to various specific and illustrative embodiments. However, it should be understood that many variations and modifications may be made while remaining within the spirit and scope of the invention. 

1. An apparatus for printing and drying an RF inkjet composition, the apparatus comprising a print cartridge adapted to travel in a line above a medium, the print cartridge comprising at least one ink nozzle, and at least one RF emitter adapted to travel along with the cartridge.
 2. The apparatus of claim 1, wherein the at least one RF emitter is positioned to activate an RF inkjet composition as it passes from the at least one print nozzle to the medium.
 3. The apparatus of claim 1, wherein the at least one RF emitter comprises an interdigitated RF probe assembly.
 4. The apparatus of claim 3, wherein the interdigitated probe assembly is incorporated into a flex circuit on the cartridge.
 5. The apparatus of claim 1 comprising a first RF emitter associated with the leading edge of the cartridge and a second RF emitter associated with the trailing edge of the cartridge.
 6. The apparatus of claim 5, wherein the first and the second RF emitters comprise interdigitated RF probe assemblies.
 7. A method of printing and drying an RF inkjet composition using the apparatus of claim 1, the method comprising ejecting an RF inkjet composition from the at least one print nozzle onto the medium and activating the RF inkjet composition using the at least one RF emitter.
 8. The method of claim 7, wherein the medium incorporates, or is coated with, an RF susceptor, the method further comprising activating the RF susceptor using the at least one RF emitter.
 9. An apparatus for printing and drying an RF inkjet composition, the apparatus comprising a print cartridge comprising at least one ink nozzle and a roller drive comprising a roller for guiding a medium under the print cartridge, the roller comprising an outer rotating cylinder and a nested inner cylinder having an RF emitter incorporated into its outer surface.
 10. The apparatus of claim 9, wherein the RF emitter comprises an interdigitated RF probe assembly.
 11. A method for printing and drying an RF inkjet composition using the apparatus of claim 9, the method comprising printing an RF inkjet composition onto a medium using the at least one ink nozzle and activating the printed ink using the RF emitter.
 12. An apparatus for printing and drying an RF inkjet composition, the apparatus comprising a print cartridge comprising at least one ink nozzle, a roller drive comprising a roller for guiding a medium under the print cartridge, a first RF emitter incorporated into the roller for directing RF energy onto the lower surface of the medium and a second RF emitter spaced apart from the roller for directing RF energy onto the upper surface of the medium.
 13. The apparatus of claim 12, wherein the second RF emitter is disposed on a plate shaped to conform to the curvature of the roller.
 14. The apparatus of claim 12, wherein one or both of the first and second RF emitters comprises an interdigitated RF probe assembly.
 15. A method for printing and drying an RF inkjet composition using the apparatus of claim 12, the method comprising printing an RF inkjet composition onto a medium using the at least one ink nozzle and activating the printed ink using the first and second RF emitters.
 16. An apparatus for printing and drying an RF inkjet composition, the apparatus comprising a print cartridge comprising at least one ink nozzle, a plate positioned to support a medium that been printed using the print cartridge and an RF emitter incorporated into the plate.
 17. The apparatus of claim 16, wherein the RF emitter comprises an interdigitated RF probe assembly.
 18. An RF-activatable sublimation ink comprising, an RF-activatable binder, a sublimation colorant and a polar carrier.
 19. The ink of claim 18, wherein the RF-activatable binder is an ionomer and the sublimation colorant is a sublimation dye.
 20. A method of sublimation printing comprising applying the ink of claim 18 to a transfer substrate and exposing the transfer substrate to RF radiation sufficient to heat the ink and cause the sublimation colorant to sublimate and deposit onto a medium.
 21. The method of claim 20, wherein the medium is selected from the group consisting of paper, textiles and non-woven materials.
 22. The method of claim 20, wherein the ink is heated to a temperature of at least about 300° C.
 23. An RF-activated impulse inkjet printer comprising at least one nozzle containing an RF inkjet composition and a RF emitter coupled to the at least one nozzle.
 24. A method of impulse printing comprising activating the RF inkjet composition of claim 23 using the RF emitter such that a bubble of vapor is created in the ink, wherein the bubble forces a drop of the ink out of the nozzle.
 25. A method for printing and drying an RF inkjet composition, the method comprising printing an RF inkjet composition onto a medium using the at least one ink nozzle and activating the printed ink using the RF emitter.
 26. An RF-activatable hot melt ink comprising, about 20 to 80 weight percent RF susceptor and about 20 to 80 weight percent colorant; wherein the ink is free of or substantially free of solvents.
 27. A method of hot melt printing comprising activating the RF susceptor to melt the ink and printing the molten ink onto a medium.
 28. An RF inkjet composition comprising an RF susceptor, a polar carrier, a crosslinking agent, and a binder selected from the group consisting of polymers polymerized from keto- or aldehyde-containing amide-functional monomers or from monomers bearing acetoacetoxy functional groups.
 29. The composition of claim 28, wherein the monomers comprise diacetone acrylamide monomers and the crosslinking agent is a di- or polyamine or a dihydrazide.
 30. The composition of claim 28, wherein the monomers comprise acetoacetoxymethacrylate or actoacetoxy ethyl acrylate monomers and the crosslinking agent is a di- or polyamine.
 31. An RF inkjet composition comprising an RF susceptor comprising carboxylic acid functional groups, a multivalent metal crosslinking agent and a solvent.
 32. The composition of claim 31, wherein the multivalent metal crosslinking agent is a zinc ammonium carbonate or zirconium ammonium carbonate.
 33. A method of inkjet printing comprising printing the ink of claim 28 onto a medium and activating the RF susceptor or RF-activatable ionomer to generate heat in the ink.
 34. A method of printing an RF inkjet composition comprising an RF susceptor, a colorant, a polar carrier and optionally a binder, the method comprising printing the ink onto a medium and activating the RF susceptor to induce a reaction between the RF susceptor, the colorant or the binder and the medium.
 35. The method of claim 34, wherein the RF susceptor is an ionomer and the reaction is a crosslinking reaction between the ionomer and the medium.
 36. The method of claim 34, wherein the ink comprises a carboxylic acid functional binder and a multivalent metal crosslinking agent and the medium is a corona treated plastic film, and further wherein the reaction occurs between the binder and the plastic film.
 37. The method of claim 34, wherein the ink comprises a cationic or amine-functional binder and the medium is a paper comprising alkyl diketene, and further wherein the reaction occurs between the binder and the alkyl diketene.
 38. The method of claim 34, wherein the ink comprises a reactive colorant and the reaction occurs between the reactive colorant and the medium.
 39. A method of printing an ink comprising a binder, a colorant, and a solvent, the method comprising either printing the ink onto a medium having an RF-activatable coating or printing the ink onto a medium and coating the printed ink with an RF-activatable coating, and activating the RF-activatable coating to induce a reaction between the colorant or the binder in the ink and the coatings.
 40. The method of claim 39, wherein the ink comprises an RF susceptor.
 41. The method of claim 39, wherein the RF susceptor comprises an RF-activatable binder.
 42. A method of inkjet printing comprising printing the ink of claim 31 onto a medium and activating the RF susceptor or RF-activatable ionomer to generate heat in the ink. 